This invention pertains to the purification or separation of gases using liquid ionic compounds.
Purified gases are necessary for many industrial processes. For example, air must be purified by removing water vapor to produce dried air for use in machinery such as spray painting equipment, dental compressors, coordinate measuring machines, process controls, HVAC systems, pneumatic controls, electronics, and the like. Furthermore, dried air is required for the preparation of dry nitrogen. Purified nitrogen, free of both water and oxygen, is used in the storage and shipping of both flowers and food, as well as in delicate scientific operations, such as gas chromatography and mass spectroscopy. Other important industrial gases that are used in purified form include helium, argon, hydrogen, oxygen, and hydrocarbons. Typically, the gas purification processes involve the removal of water, carbon dioxide, or other component gases which may interfere with the end-use dependent application of the purified gas.
The natural gas commonly used in nearly every household originates in underground sedimentary rock formations. Natural gas may contain a variety of impurities including carbon dioxide (CO2) and water. It is desirable to remove these two impurities for several reasons. Water may form hydrocarbon hydrates, possibly forming solids, that can plug pipelines and pumping equipment. These hydrates are an exceptional concern in cold climates or in high-pressure systems where solid formation may be more likely. In addition, the presence of CO2 tends to decrease the heating value of natural gas. Finally, the combination of CO2 and water impurities in natural gas may form carbonic acid which is corrosive to pumping equipment and the pipeline used for transporting the natural gas to storage facilities or end-users. Economic benefit is realized by removing these impurities, and by doing so as close to the well-head as possible.
Current methods of gas purification include the use of serial gas/liquid absorbers or gas/solid adsorbers. Gas/liquid absorbers may include pure liquids or liquid solutions that preferentially partition components of the gas. Gas/solid adsorbers may include substances, such as sodium bicarbonate, and the like, that preferentially remove certain compounds based upon affinity, or molecular sieves that differentiate the various gas components on the basis of molecular size or other physical property. The liquid or solid material employed in these separators is known as a mass separating agent (MSA) and advantageously exhibits a differential affinity for one or more of the gas components. MSAs may also be chosen, for example based on their stability to extreme environments, such as extremes in temperature or pressure, stability to certain organic solvents, and stability to pH extremes. Furthermore, MSAs may be added directly to a process stream to enable separation.
Other separation schemes may also be employed, such as the use of inorganic membranes, polymeric membranes, supported liquid membranes, and the like. Gases may also be separated in cryogenic processes.
Serial gas/liquid and gas/solid separators may be configured in linear arrays of absorption and adsorption units. These arrays are designed to remove a plurality of gaseous impurities by contacting a gas stream with several liquid or solid MSAs, each designed to remove preferentially at least a portion of these gaseous impurities, thereby producing an effluent gas stream enriched in the remaining gaseous components. In such processes, the liquid or solid MSA becomes loaded with the gaseous impurity. In an alternative arrangement, the liquid or solid MSA may preferentially take up the desired gas component for recovery later, thus producing an effluent gaseous stream of impurities. The desired component may be released from the MSA in another step of the process. It is appreciated that the process design selected for removal of impurities or the alternative collection of a desired component in a given gas purification procedure will depend upon several factors, including the selectivity exhibited by the MSA for particular gas components, the ease with which the desired component may be recovered from the MSA, and others.
In systems where impurities are removed by selective absorption, it is often the case that multiple absorbers are used, one for each gas component impurity. For example, at present, two absorbers are required to remove CO2 and water from natural gas. Carbon dioxide can be selectively removed with a gas/liquid absorption unit charged with an aqueous amine solution, such as mono- or diethanolamine. These amines form carbamates upon reaction with CO2, and these carbamates preferentially partition into the aqueous liquid stream. Similarly, water may be removed by preferential absorption with a gas/liquid absorption unit charged with an ethylene glycol liquid stream. In addition, water may be preferentially removed with polymeric membrane modules.
As many gas purification processes require the removal of more than one impurity, current conventional gas/liquid absorbers employ a separate absorption unit for removing each impurity. For example, one unit designed to remove carbon dioxide and an additional unit designed to remove water may be used as described above for natural gas. In addition, volatile components present in the absorbers, such as the amines used for CO2 absorption, often evaporate into the gas stream. Thus, the removed CO2 impurity may be replaced by the amine absorbing component. The resulting amine contamination is typically removed via condensation by means of a cold trap, and may be returned to the absorbing unit. However, such removal requires additional components added to the purification system. The requirement for multiple absorption units along with additional purification steps to remove subsequently released MSA can increase process time and operating costs. Furthermore, such complex systems may preclude near well-head implementation in deference to a centralized system for purification, and further increase the overall cost of goods resulting from the increased cost of transporting impure material. Finally, once exhausted, traditional absorbers must be replaced and few options are available for recovery or regeneration of spent MSAs, thereby adding replacement and disposal costs.
Liquid ionic compounds (LICs), often called xe2x80x9cionic liquidsxe2x80x9d are essentially non-volatile, having immeasurably low vapor pressures; they are not volatilized into the purified gas stream. Their low vapor pressure minimizes loss of absorbing material during use and provides a simple mechanism for regeneration, such as by distillation, evacuation, or by extraction with a supercritical fluid, such as supercritical carbon dioxide.
As described herein, in one embodiment, the LIC selectively solubilizes impurities, leaving the desired gas in the gas stream. It is appreciated that in variations of the methods described herein, the LIC may selectively solubilize the desired gas component, leaving the impurities behind in the effluent stream. In such variations, recovery of the desired material may be accomplished by processes analogous to the regeneration mechanisms described above. Distillation, evacuation, or extraction with a supercritical fluid, and the like, will regenerate the LIC and simultaneously recover the desired purified gas; recovery rates greater than 90% are not unexpected.
Moreover, LICs can be tailored for specific needs, allowing a single absorption unit to be used for the removal of more than one impurity, depending on the relative solubilities and/or diffusibilities in the LIC of the desired gas and the impurities.
Furthermore, LICs may be prepared by simple and relatively inexpensive methods. Therefore, purification systems designed around LICs may be more amenable to near-wellhead processes in anticipation of their reduced operating costs. Finally, their exceedingly low vapor pressures make LICs environmentally friendly, during both use and regeneration, since both MSA loss and waste associated with MSAs may be minimized.
Thus, in one embodiment a method is provided for purifying a gaseous mixture by contacting the gaseous mixture with a liquid ionic compound. In an illustrated embodiment, natural gas containing impurities such as water and carbon dioxide may be advantageously processed using LICs to provide purified natural gas for various industrial uses. In an illustrated example, the LICs preferentially absorb those impurities when contacted with the crude gas stream. Similarly, in another illustrated example, in the first step of producing purified nitrogen from air, impurities such as water and carbon dioxide are removed using an LIC. Subsequent separation of the oxygen and nitrogen may be performed by cryogenic distillation. In another example, commercial argon is also produced via the cryogenic distillation of air. In some processes, liquefaction and distillation are used to produce a low-purity crude argon product. Further purification to a higher-purity commercial product by removing oxygen with a LIC is contemplated by the present invention.
It is appreciated that a gas may be contacted with the liquid ionic compound by conventional means known by those of ordinary skill in the art. Once the liquid ionic material contacts the gas, certain components such as carbon dioxide and water are extracted from the gas owing to the solubilities exhibited by various gas components in the LIC. LICs of the present invention include, but are not limited to, quaternary imidazolium salts, and quaternary aromatic 5- and 6-membered-ring heterocycles such as imidazolium salts, pyridinium salts, and the like. Specific examples include, but are not limited to, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-octyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium nitrate, 1-octyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium ethylsulfate, and N-butylpyridinium tetrafluoroborate.